1. Field of the Invention
The present invention relates to aluminum door beams used in reinforcing members for doors of vehicles, such as automobiles and trucks. The door beam is arranged in a door to absorb the shock from a collision in the side direction and to secure safety of passengers.
2. Description of the Related Art
Recently, the global environment has been regarded as being of worldwide importance. For example, regulations for reducing gas emissions including carbon dioxide from automobiles have been strengthened in many countries in order to suppress global warming. Accordingly, lightweight automobiles have been in rapid development.
A door beam for an automobile is attached to the interior of a door in order to absorb the shock from a collision. A typical conventional material used is steel, for example, high-tensile steel of 150 kgf/mm.sup.2 grade. In recent years, however, the use of aluminum extrusions has been investigated in view of achievement of a lightweight automobile.
Door beams for automobiles (also referred to as impact beams, impact bars, guard bars, or door side beams) are required to have high energy absorbability to soften the shock from a collision. For example, Federal Motor Vehicle Safety Standard (FMVSS) defines criteria of the bending strength and absorbed energy to a load applied from the side of a vehicle. At laboratory tests, these bending properties are evaluated by a three-point bending strength test simulating side collision of a vehicle as shown in FIG. 2A, in which a door beam is supported at the two ends and a load is applied to the center.
FIG. 2B is a typical schematic load (P) vs. displacement (.delta.) curve in the three-point bending test shown in FIG. 2A. FIG. 2B shows that the load reaches a maximum value as the displacement increases, and then it decreases at a further displacement because of overload buckling of the aluminum beam. In general, it is preferred that the maximum load be larger and the displacement when the buckling occurs be larger, that is, the energy absorption be larger, as shown by a solid line in FIG. 3. The energy absorption corresponds to the area represented by hatched lines in the load (P) vs. displacement (.delta.) curve of FIG. 2B.
Stricter properties have been required for door beams being highly conscious of safety, that is, improvements in maximum load and energy absorption without an increase in the weight have been required. For example, in a three-point bending test under a specified condition for door beams, a current required level of the maximum load is 1,300 kg, which is considerably higher than the conventional level 1,100 kg.
Recently, door beams have been applied to compact cars having short doors. Since the distance (L) between the two ends in FIG. 2A is short, in collision of compact cars, a small displacement (.delta.) causes a larger bending curvature. Thus, rupture will occur more readily with a small displacement.